Biodiesel has been the subject of much investigation as an alternative for petroleum diesel fuel. As used herein, the term “biodiesel” refers to an ester-based fuel oxygenate that is derived from a biological source. The biodiesel fuel is used as an alternative for, or as an additive to, petroleum diesel fuel in automobiles or other vehicles. The biodiesel fuel is conventionally produced from a triglyceride starting material or a fatty acid starting material by a transesterification reaction or an esterification reaction, respectively. Generally, the triglyceride is reacted, or transesterified, with an alcohol to produce glycerol (also known as glycerin) and a corresponding alkyl ester of the triglyceride. Similarly, the fatty acid is reacted, or esterified, with an alcohol to produce a corresponding alkyl ester of the fatty acid. The triglyceride or fatty acid feedstock materials are available from various sources, such as from pure or used fats or oils. These fats or oils are typically hydrophobic and water-insoluble. In addition to including the triglyceride or fatty acid, the fats or oils include free fatty acids, phospholipids, sterols, water, dirt, detergents, polar compounds, or other impurities. These sources of the fats or oils are too viscous and their freezing points too high to be used directly as the biodiesel fuel and, therefore, the triglycerides or fatty acids are transesterified or esterified to produce the corresponding alkyl ester, which has a lower viscosity than that of the source of the feedstock material. The transesterification of the triglyceride (or the esterification of the fatty acid) is conducted with an excess of the alcohol in the presence of a catalyst. As the reaction proceeds, two phases form. One phase includes the alkyl ester and the other phase includes the glycerol. The two phases are allowed sufficient time to settle before additional processing is conducted to purify the alkyl ester from the glycerol.
Currently, most biodiesel is made from soybean oil, methanol, and an alkaline catalyst. However, there are large amounts of waste fats and oils that are unsuitable for human consumption that could be converted to biodiesel at lower cost. The problem with processing these waste oils is that they often contain large amounts of free fatty acids that cannot be converted to biodiesel using an alkaline catalyst. These free fatty acids react with the alkaline catalyst to produce soaps that inhibit the separation of the biodiesel, glycerin, and wash water. An acid-catalyzed pretreatment with an alcohol, such as methanol, has been used to convert these high free fatty acid feedstocks to methyl esters that do not form soaps. However, this pretreatment leaves behind acid and water that interferes with the alkaline transesterification. The free fatty acids could first be extracted using a solvent, such as an alcohol, and the purified triglyceride would then undergo base-catalyzed transesterification. The fatty acid could be recovered from the solvent and then be esterified using an acid catalyst and methanol to produce the alkyl ester, sold, or simply discarded. Additionally, in some cases, the biodiesel product contains unreacted free fatty acids, which cause the product to be off spec. Again, these free fatty acids can be extracted using a solvent such as an alcohol.
Further, in food and chemical processes, some fats and oils need to have impurities such as, for example, free fatty acids, sugars, and non-oil lipids, removed to improve their quality. This too can be accomplished by extracting the impurities with a solvent, such as an alcohol. The extraction process results in the formation of a waste product including the solvent contaminated with the impurities removed from the fat or oil.
Removing the free fatty acids from a fat or oil results in additional complexity of recovering and recycling the solvent, which has proven to be problematic. Particularly, the solvent, is typically removed by distillation, evaporization, lyophilization, use of an inert gas, or use of a drying agent, such as a molecular sieve.
Because these conventional techniques consume considerable amounts of energy or use expensive and toxic compounds, efficient methods of recovering a solvent used to extract polar impurities, such as free fatty acids and water, from fat, oil, or other feedstock stream are desired.